Find maximum operations to reduce N to 1
Given two numbers A and B ( A and B can be up to 106 ) which forms a number N = (A!/B!). The task is to reduce N to 1 by performing maximum number of operations possible.
In each operation, one can replace N with N/X if N is divisible by X.
Find the maximum number of operations that can be possible.
Examples:
Input : A = 6, B = 3 Output : 5 Explanation : N is 120 and the divisors are 2, 2, 2, 3, 5 Input : A = 2, B = 1 Output : 1 Explanation : N is 2 and the divisor is 2.
Observe that factorization of number A!/B! is this same as factorization of numbers (B + 1)*(B + 2)*…*(A – 1)*A.
Also, the number of operations will be maximum if we divide N with only with it’s prime factors. So, in other words we need to find the count of prime factors of N including duplicates.
Let’s count the number of prime factors in the factorization of every number from 2 to 1000000.
First, use Sieve of Eratosthenes to find a prime divisor of each of these numbers. Then we can calculate the number of prime factors in the factorization of a using the formula:
primefactors[num] = primefactors[num / primediviser[num]] + 1
Now, one can use prefix sum array for prime factors and then answer for the sum on an interval [A, B].
Below is the implementation of the above approach:
C++
// CPP program to find maximum // number moves possible #include <bits/stdc++.h> using namespace std; #define N 1000005 // To store number of prime // factors of each number int primeFactors[N]; // Function to find number of prime // factors of each number void findPrimeFactors() { for (int i = 2; i < N; i++) // if i is a prime number if (primeFactors[i] == 0) for (int j = i; j < N; j += i) // increase value by one from // it's preveious multiple primeFactors[j] = primeFactors[j / i] + 1; // make prefix sum // this will be helpful for // multiple test cases for (int i = 1; i < N; i++) primeFactors[i] += primeFactors[i - 1]; } // Driver Code int main() { // Generate primeFactors array findPrimeFactors(); int a = 6, b = 3; // required answer cout << primeFactors[a] - primeFactors[b]; return 0; } |
Java
// Java program to find maximum // number moves possible import java.io.*; class GFG { static int N= 1000005; // To store number of prime // factors of each number static int primeFactors[] = new int[N]; // Function to find number of prime // factors of each number static void findPrimeFactors() { for (int i = 2; i < N; i++) // if i is a prime number if (primeFactors[i] == 0) for (int j = i; j < N; j += i) // increase value by one from // it's preveious multiple primeFactors[j] = primeFactors[j / i] + 1; // make prefix sum // this will be helpful for // multiple test cases for (int i = 1; i < N; i++) primeFactors[i] += primeFactors[i - 1]; } // Driver Code public static void main (String[] args) { // Generate primeFactors array findPrimeFactors(); int a = 6, b = 3; // required answer System.out.println (primeFactors[a] - primeFactors[b]); } } // This code is contributed by jit_t. |
Python3
# Python3 program to find maximum # number moves possible N = 1000005 # To store number of prime # factors of each number primeFactors = [0] * N; # Function to find number of prime # factors of each number def findPrimeFactors() : for i in range(2, N) : # if i is a prime number if (primeFactors[i] == 0) : for j in range(i, N, i) : # increase value by one from # it's preveious multiple primeFactors[j] = primeFactors[j // i] + 1; # make prefix sum this will be # helpful for multiple test cases for i in range(1, N) : primeFactors[i] += primeFactors[i - 1]; # Driver Code if __name__ == "__main__" : # Generate primeFactors array findPrimeFactors(); a = 6; b = 3; # required answer print(primeFactors[a] - primeFactors[b]); # This code is contributed by Ryuga |
C#
// C# program to find maximum // number moves possible using System; class GFG { static int N = 1000005; // To store number of prime // factors of each number static int []primeFactors = new int[N]; // Function to find number of prime // factors of each number static void findPrimeFactors() { for (int i = 2; i < N; i++) // if i is a prime number if (primeFactors[i] == 0) for (int j = i; j < N; j += i) // increase value by one from // it's preveious multiple primeFactors[j] = primeFactors[j / i] + 1; // make prefix sum // this will be helpful for // multiple test cases for (int i = 1; i < N; i++) primeFactors[i] += primeFactors[i - 1]; } // Driver Code static public void Main () { // Generate primeFactors array findPrimeFactors(); int a = 6, b = 3; // required answer Console.WriteLine(primeFactors[a] - primeFactors[b]); } } // This code is contributed by ajit |
PHP
<?php // PHP program to find maximum // number moves possible $N = 10005; // To store number of prime // factors of each number $primeFactors = array_fill(0, $N, 0); // Function to find number of prime // factors of each number function findPrimeFactors() { global $N,$primeFactors; for ($i = 2; $i < $N; $i++) // if i is a prime number if ($primeFactors[$i] == 0) for ($j = $i; $j < $N; $j += $i) // increase value by one from // it's preveious multiple $primeFactors[$j] = $primeFactors[(int)($j / $i)] + 1; // make prefix sum // this will be helpful for // multiple test cases for ($i = 1; $i < $N; $i++) $primeFactors[$i] += $primeFactors[$i - 1]; } // Driver Code // Generate primeFactors array findPrimeFactors(); $a = 6; $b = 3; // required answer print(($primeFactors[$a] - $primeFactors[$b])); // This code is contributed by chandan_jnu ?> |
Output:
5
